The purity of aluminum oxide for smelting, produced by the Bayer process digestion of bauxite, depends upon the type and quality of the bauxite. Certain minor impurities in bauxite are carried through the bauxite refining process and ultimately contaminate the metallic aluminum. A maximum zinc content of 0.03 weight percent can be tolerated in certain alloys. Theoretically, a zinc content of 0.03 weight percent in the metal is derived from an aluminum oxide containing about 0.02 weight percent ZnO. To produce satisfactory aluminum metal, the CuO and ZnO contents of aluminum oxide should generally be held to less than about 0.015 and 0.023 weight percent, respectively.
Methods for removal of copper and zinc from the Bayer process liquor are known in the prior art. However, each of these prior art methods suffers from one or more serious disadvantages making it less than entirely suitable for its intended purpose. For example, the following patents each disclose the lowering of zinc content in the Bayer liquor by addition of sodium sulfide to the liquor: U.S. Pat. Nos. 2,885,261, 3,445,186, 3,469,935, 4,282,191 and British Pat. No. 1,373,843.
Due to the amphoteric properties of zinc, there are very few Zn.sup.++ ions in the alkaline green liquor. Nearly all of the zinc is present as zincate ions, ZnO.sub.2.sup.= or Zn(OH).sub.4.sup.=. The term "zinc species" as used herein refers to all forms of dissolved zinc present in industrial sodium aluminate solutions, including Zn.sup.++, ZnO.sub.2.sup.= and Zn(OH).sub.4.sup.= ions. In a plant liquor, the presence of aluminate and carbonate ions may lead to formation of colloidal zinc oxide or hydroxide. In addition, the Bayer liquor includes other ions which will also react with sulfide ions. Therefore, the reaction between zinc species and sulfide ions in the Bayer liquor is by no means stoichiometric. Precipitation of zinc sulfide by addition of sodium sulfide requires an excess of sulfide over the stoichiometric amount. This excess sulfide is an undesirable contaminant in the production of alumina. When the sulfide is converted to sulfate, the sulfate-containing liquor is corrosive to metal containers.
U.S. Pat. No. 2,885,261 reported that addition of sodium sulfide to the Bayer liquor presented another problem. More than half of the sodium sulfide charge can be expected to remain in the system and eventually be oxidized to sodium sulfate. Therefore, addition of sodium sulfide to a plant Bayer liquor stream for an extended period can result in a rapid increase of liquor contamination approaching approximately 50 grams per liter. This high sodium sulfate contamination of the liquor stream imposed serious operating difficulties, such as slowdown of aluminum hydroxide precipitation and production of a slightly finer product. It was claimed that the problem could be overcome by adding sodium sulfide to only about one-half of the total liquor stream.
Another publication has indicated that high sodium sulfide concentration in the Bayer liquor stream causes high iron levels in aluminum hydroxide precipitated from the stream. See V. V. Grachev and S. I. Kuznetsov, "The Influence of Various Sulfur Compounds on the Solubility of Iron in Alkaline and Aluminate Solutions," Tsvetnaya Metallurgiya, Volume 3, 1974, pages 63-67.
A filtration process has previously been used to remove suspended solids and colloidal iron from Bayer liquor. See L. K. Hudson, "Alcoa's Process Filter," Light Metals, Volume 3, 1974, pages 737-744. Sand particles in the filter used in this process contained Fe.sub.2 O.sub.3. However, the ability of such filter to remove copper and zinc species from solution has not previously been discovered.
In view of the numerous difficulties that may arise when soluble sulfides are added to the Bayer liquor streams to precipitate zinc or copper, there is a need for a process that will remove zinc or copper from an aluminate solution without adding sulfide to the solution.
It is a principal object of the present invention to provide a process for removal of copper and zinc species from a sodium aluminate solution without increasing the sulfide content of the solution.
It is a related object of the invention to provide a process for removal of copper and zinc species from a sodium aluminate solution that is economically suitable for practice on an industrial scale.
Additional objects and advantages of the present invention will become apparent to persons skilled in the art from the following specification.